The invention relates to a control device in a helical screw rotor machine for elastic fluid comprising a casing provided with a barrel portion defined by intersecting bores with parallel axes located between axially spaced end walls of the casing and having inlet and outlet ports communicating with the bores, each rotor having helical lands and intervening grooves, the male rotor lands having their major portions radially outside the pitch circle of the male rotor and with convexly curved flanks, and the female rotor lands having their major portions radially inside the pitch circle of the female rotor and with concavely curved flanks. The lands and grooves of the rotors are intermeshing to form with the bores chevron-shaped working chambers each comprising communicating portions of a male and a female groove, the chevron-shaped chambers being defined at their base ends by the stationary end walls of the casing and at their apex ends by the intermeshing lands of the rotors, the volumes of said chevron-shaped chambers being varied as the rotors revolve. A valve means is provided which comprises a first and a second axially aligned and separately operated valve member for regulating the quantity of elastic fluid passing through the machine and the pressure ratio thereof, respectively, said valve members being slidable in an axially extending recess formed in the casing so as to open directly into the bores, the valve members having an inner face of shape complementary to the envelope of that portion of the bores confronted by the opening of the recess and adapted to control an opening venting the working chamber to the inlet port and the area of the outlet port.
It is known to regulate the capacity of a compressor of this type by using a single slidable valve member by means of which the initial and final volumes of the chevron-shaped working chambers are varied. Thus, when the capacity is varied, the built-in pressure ratio or volume ratio is varied as well.
This means for instance that when the capacity is decreased to a low value, the built-in pressure ratio or volume ratio is increased at the same time to an unsuitably high value, resulting in a reduced efficiency.
In order to at least reduce this drawback, it is suggested in U.S. Pat. No. 3,088,659, FIG. 8, to utilize a valve means comprising first and second axially aligned and separately operated valve members one of which--called a movable slide stop--is used for regulating the capacity and the other--called a slide valve is used for regulating the pressure ratio or volume ratio. The slide stop of U.S. Pat. No. 3,088,659 opens a recirculation slit to the inlet port in which case the machine is run at part load, i.e. part of the air or gas will be bled from the working chambers back to the inlet port via the recirculation slit. The volumetric flow drawn by the machine through the inlet port is dependent on the size of the recirculation slit and the position of the slide valve. Both of these factors will also affect the matching of the inner compression pressure to the external pressure ratio. Thus, the size of the recirculation slit as well as the position of the slide valve both affect the capacity of the machine. Different part-load capacities can be achieved with different combinations of slit size and slide valve position but only one combination in this range provides the best efficiency.
A special disadvantage of the movable slide stop occurs at high pressure ratios. Under such conditions the slide valve is moved far towards the outlet end. The recirculation gap between the slide stop and the slide valve will then be locate relatively late in the compression process. Losses will be incurred due to the fact that the compression work has been done on the gas which is recirculated back to the inlet port through the recirculation slit. These losses obviously occur in all machines of this type provided with slide valve control. However, in this particular case the losses will be higher than usual since the gap is positioned so late in the compression process.
The object of the present invention is to eliminate these drawbacks and to make it possible to run the machine under at least two different part-load conditions with best efficiency. This is especially important in refrigeration applications. In refrigerated cargo ships, for instance, different cargoes must be maintained at different temperatures. In case of fruit shipments, the temperature may even have to be varied in accordance with a certain pattern during the course of the journey. The evaporation temperature of the refrigeration system must apparently also differ on different occasions.
In freezer, refrigeration or air conditioning installations employing air-cooled condensers, the condensation temperature of the system varies with the season of the year so that it is high during the summer and low in winter. In the case of water-cooled condensers the water temperature and thus also the condensation temperature sometimes vary with the season of the year, depending on the source of the cooling water.
In the case of heat pumps, the energy-absorber temperature often varies with the season of the year.